1. Field of the Invention
The present invention relates generally to the field of medical implants. More specifically, the present invention relates to a medical implant of the type having an oscillator monitoring circuit for monitoring the functioning of an oscillator in the medical implant, and a method for monitoring the functioning of an oscillator in a medical implant, such as a heart stimulator.
2. Description of the Prior Art
For modern electronic circuits, it is generally essential to provide an accurate clocking signal in order to synchronize the different electronic functions of the circuit. Generally, a single master timing source, such as an oscillator, is used to produce a periodic signal at a fixed frequency. An accurate clock signal is imperative for a proper functioning of the electronic circuit. If the frequency of the periodic signal deviates from its predetermined frequency, the circuit will not function in the intended manner.
Within the field of medical implants, i.e. heart stimulators, the master timing source is generally an oscillator. Heart stimulators are life supporting, therapeutic medical devices that are surgically implanted and remain within a person's body for years. Thus, a need exists for monitoring and checking the master oscillator of the heart stimulator to determine if the frequency of the oscillator periodic signals deviates from its predetermined clock frequency and to handle such a deviation if it occurs.
U.S. Pat. No. 4,590,941 discloses a cardiac pacer having stimulating logic for producing an output stimulating signal, the stimulating logic including a crystal oscillator and a digital circuit serving as the pacing logic of the pacer. The pacer further has a continuously operating RC oscillator and a frequency checking circuit. The RC oscillator is an emergency oscillator continuously producing an output at a predetermined acceptable frequency and a predetermined pulse width. The crystal frequency is tested by the frequency checking circuit using the output of the RC oscillator. The pacer further has a gating means for substituting the output of the RC oscillator for the output of the stimulating logic upon detection of failure of the crystal oscillator.
Hence, the reference parameter used for continuously testing the frequency of the crystal oscillator is the output frequency of the RC oscillator. This requires a continuous operation of the RC oscillator. Furthermore, the frequency checking circuit requires a reliable output from the RC oscillator in order to provide a safe and accurate result. Otherwise, the frequency of the crystal oscillator could be considered to deviate from the correct frequency when, in fact, it is the frequency of the RC oscillator that deviates from the predetermined frequency.
It is an object of the present invention to provide a method, and a medical implant using the method, for detecting with improved reliability a frequency deviation of the output frequency of an oscillator in a medical implant.
The above object is achieved in accordance with the principles of the present invention in a method and circuit for monitoring an oscillator in a medical implant wherein at least one physiological parameter, having a time component, is obtained from a subject in whom the medical implant is implanted, and wherein an electrical signal is generated that is related to the time component, and wherein this electrical signal is used as an indicator of a deviation of the functioning of the oscillator from an intended or specified functioning of the oscillator.
The invention is based on using a physiological parameter emanating from the human body for monitoring the status of the output frequency of a timing circuit in a medical implant. Hence, deviations in the output frequency of the timing circuit are detected by using the physiological parameter as a reference. Preferably, the timing circuit is an oscillator.
By using a physiological parameter for detecting a deviation in the output frequency of an oscillator, use is made of a parameter that is always present, i.e. the physiological parameter can be used for detecting a frequency deviation regardless of whether there is a fault in the electronic circuitry or not. This might not always be the case when a parameter obtained from within the electronic circuitry is used for the deviation detection. In fact, a deviation in the output frequency of a main oscillator in an electronic circuit, can cause resulting effects in the electronic circuitry making components within the circuitry unsuitable, or unusable, for providing a reference parameter for the monitoring.
Furthermore, the problem described in relation to prior art regarding the risk of misinterpreting the result, i.e. the output frequency one oscillator being considered to deviate when the deviation occurs in the output frequency of the other oscillator, is eliminated according to the present invention. This is due to the fact that the monitoring of an oscillator does not involve any other oscillator that might be present in the medical implant.
The physiological parameter used for monitoring the output frequency of the oscillator contains a time component. The time component of the physiological parameter is used for monitoring deviation of the frequency from a permitted value or range.
As is known to a person skilled in the art, any physiological parameter varies over time. Therefore, an exact time value can not be obtained from a physiological parameter. However, the typical oscillator used as a main oscillator in a medical implant is a crystal oscillator, which is calibrated before encapsulation by mechanical trimming. It is well known that, if the output frequency of a crystal oscillator deviates from its intended frequency, it deviates drastically, the output frequency for instance changing to zero or multiples of the intended frequency. Thus, a physiological parameter can be used for monitoring the status of an oscillator, even though the parameter varies slightly over time.